Conventionally, simple transformer which steps down a single-phase 100 V in Japan and a diode rectifier are used in order to charge a secondary battery like a vehicular battery. For large-capacity charging, various schemes for power conversion from AC power to DC power are made in practical use. High power factor and high efficiency are desired for such power conversion schemes, and reduction of the number of component parts and simplification of controlling are also desired.
Charging can be performed by a PFC circuit which is the latest power conversion technology with a PWM converter and a flyback circuit using semiconductor switches that can be switched off, such as a power MOSFET and an IGBT (see patent literatures 1, 2, 3, and 4). Moreover, various systems are also proposed, such as a system of efficiently charging plural vehicles in consideration of cost-effectiveness (see patent literature 2), a system of performing rapid charging efficiently regardless of a change in an environment (see patent literature 3), and a system of performing charging from a three-phase AC source (see patent literature 4). However, all of those systems perform hard switching for power conversion, thus having a large switching loss. Moreover, controlling of charging power regardless of the voltage value of a system voltage and the frequency thereof rises a problem from the standpoint of the system stability.
High-speed semiconductor switches are desirable for power conversion, but as a circuit technology, a soft switching technology which zeroes either one of or both of a voltage and a current at on/off time is preferable from the standpoint of not only the conversion efficiency but also its capability of reducing any generation of noises, so that such a technology is an important solution.
There is proposed a switch (hereinafter, Magnetic Energy Recovery Switch (MERS)) which can perform on/off control on currents in both forward and backward directions through gate controlling only using four reverse conductive type semiconductor devices (hereinafter, reverse conductive type semiconductor switches) having no reverse blocking capability, stores magnetic energy possessed by a current in a capacitor when the current is cutoff, and performs discharging to a load side through a semiconductor device where a on-gate signal is applied to regenerate a current, thereby accomplishing the bidirectionality of currents and regenerating magnetic energy possessed by a circuit without any loss (see patent literature 5). Moreover, there is proposed an AC/DC power converter which performs switching in synchronization with an AC power source using such Magnetic Energy Recovery Switch (MERS) (see patent literature 6).
Furthermore, there is proposed a device for performing stable power-flow control regarding power reception from a system (see patent literature 7).
Patent Literature 1: Unexamined Japanese Patent Application KOKAI Publication No. 2008
Patent Literature 2: National Patent Application Publication No. 2007-535282
Patent Literature 3: Unexamined Japanese Patent Application KOKAI Publication No. 2007-049828
Patent Literature 4: Unexamined Japanese Patent Application KOKAI Publication No. 2007-097341
Patent Literature 5: Japanese Patent No. 3634982
Patent Literature 6: International Publication No. WO2008/096664
Patent Literature 7: International Publication No. WO2005/067117